In the course of daily life, one typically regards objects located at different distances from the eye. To selectively focus on such objects, the focal length of the eye's lens must change. In a healthy eye, this is achieved through the contraction of a ciliary muscle that is mechanically coupled to the lens. To the extent that the ciliary muscle contracts, it deforms the lens. This deformation changes the focal length of the lens. By selectively deforming the lens in this manner, it becomes possible to focus on objects that are at different distances from the eye. This process of selectively focusing on objects at different distances is referred to as “accommodation.”
As a person ages, the lens loses plasticity. As a result, it becomes increasingly difficult to deform the lens sufficiently to focus on objects at different distances. This condition is known as presbyopia. Refractive errors caused by such conditions as hyperopia, myopia, as well as aberrations due to irregularities in the eye (e.g., in the cornea or in the natural crystalline lens) can also cause problems in one's ability to focus on an object. To compensate for this loss of function, it is useful to provide different optical corrections for focusing on objects at different distances. Some restoration of focusing ability for some distances can be provided by spectacles or contact lenses.
There are also a variety of disorders that degrade the ability of the eye to function properly. These include vitreoretinal disorders, lenticular disorders, corneal disorders, and glaucomatous states. Some treatments to some of these types of disorders involve surgical intervention. For example, a common disorder involves progressive clouding of the natural crystalline lens resulting in the formation of what is referred to as a cataract. A common practice used to treat a cataract is surgically removing the cataractous natural crystalline lens and implanting (in the “aphakic” patient) an artificial intraocular lens into the empty lens bag to replace the natural crystalline lens. After cataract surgery, the corneal incision (and/or limbal and scleral incisions) can potentially induce optical aberrations due to altered corneal curvature and topography. Intraocular lenses can also be used for a “phakic” patient who still has a natural crystalline lens.